ELECTROSTATIC AND ELECTRICAL TRANSPORT ANALYSIS OF NANOMATERIALS AND NUMERICAL METHODS DEVELOPMENT

The nanotechnology today is continuously boosting the application of nanostructured materials in the development and innovation of electronic devices, such as Nano &ndash Electromechanical Systems (NEMS), electrical transistors, thermoelectric devices, and solar cells. Due to the size miniaturization, quantum mechanical effects play important roles in the performance of such devices. To correctly capture the quantum mechanical effects and understand how these effects influence the electrostatic and electrical transport properties of nanomaterials, efficient and accurate computational models are highly desirable. Currently, the commonly used model for electrostatic analysis of nanoscale devices is based on self &ndash consistent solution of the effective &ndash mass Schroedinger equation coupled with the Poisson equation. However, a major drawback of this model is its inefficiency to simulate systems with large Degrees of Freedom (DOFs). To reduce the computational cost, in this thesis, two Component Mode Synthesis (CMS) approaches, namely the fixed &ndash interface CMS and the free &ndash interface CMS, are incorporated into the Schroedinger &ndash Poisson model to speed up the electrostatic analysis in nanostructures. The new model is employed to analyze the quantum electrostatics in both nanowires and FinFETs. Numerical results demonstrate the superior computational performance in terms of efficiency and accuracy. In addition to the electrostatic analysis, carrier transport in nanostructures with perturbation from quantum effects also merits careful consideration. Among the computational models developed for quantum mechanical carrier transport analysis, the Non &ndash Equilibrium Green &rsquo s Function (NEGF) coupled with Poisson equation has gained vast application in both ballistic and diffusive transport analysis of nanodevices. In this thesis, the NEGF model is expanded to include mechanical strain and carrier scattering effects. Two important multiphysics systems are investigated in this work. We first study the effect of mechanical strain on the electrical conductivity of Si/Si 1 &minus x Ge x nanocomposite thin films. The strain effect on the bandstructures of nano &ndash thin films is modeled by a degenerate two &ndash band k · p theory. The strain induced bandstructure variation is then incorporated in the NEGF &ndash Poisson model. The results introduce new perspectives on electrical transport in strained nano &ndash thin films, which provides useful guidance in the design of flexible electronics. Secondly, nanoporous Si as an efficient thermoelectric material is studied. The Seebeck coefficient and electrical conductivity of nanoporous Si are computed by using the NEGF &ndash Poisson model with scatterings modeled by Buttiker probes. The phonon thermal conductivity is obtained by using a Boltzmann Transport Equation (BTE) model while the electron thermal conductivity is captured by the Wiedemann &ndash Franz law. The thermoelectric figure of merit of nanoporous Si is computed for different doping density, porosities, temperature and pore size. An optimal combination of the material design parameters is explored and the result proves that nanoporous Si has better thermoelectric properties than its bulk counterpart. In the electrical transport analysis of nanomaterials, we found that the standard NEGF &ndash Poisson model using the Finite Difference (FD) method has a high computational cost, and is inapplicable to devices with irregular geometries. To overcome these difficulties, an accelerated Finite Element Contact Block Reduction (FECBR) method is developed in this thesis. The performance of the accelerated FECBR is evaluated through the simulation of two types of electronic devices: taper &ndash shaped DG &ndash MOSFETs and DG &ndash MOSFETs with Si/SiO 2 interface roughness. Numerical results show that the accelerated FECBR can be applied to model ballistic carrier transport in devices with multiple leads, arbitrary geometry and complex potential profile. The accelerated FECBR significantly improves the flexibility and efficiency of electrical transport analysis of nanomaterials and nanodevices

Simulation of multigate SOI transistors with silicon, germanium and III-V channels

In this work by employing numerical three-dimensional simulations we study the electrical performance and short channel behavior of several multi-gate transistors based on advanced SOI technology. These include FinFETs, triple-gate and gate-all-around nanowire FETs with different channel material, namely Si, Ge, and III-V compound semiconductors, all most promising candidates for future nanoscale CMOS technologies. Also, a new type of transistor called “junctionless nanowire transistor” is presented and extensive simulations are carried out to study its electrical characteristics and compare with the conventional inversion- and accumulation-mode transistors. We study the influence of device properties such as different channel material and orientation, dimensions, and doping concentration as well as quantum effects on the performance of multi-gate SOI transistors. For the modeled n-channel nanowire devices we found that at very small cross sections the nanowires with silicon channel are more immune to short channel effects. Interestingly, the mobility of the channel material is not as significant in determining the device performance in ultrashort channels as other material properties such as the dielectric constant and the effective mass. Better electrostatic control is achieved in materials with smaller dielectric constant and smaller source-to-drain tunneling currents are observed in channels with higher transport effective mass. This explains our results on Si-based devices. In addition to using the commercial TCAD software (Silvaco and Synopsys TCAD), we have developed a three-dimensional Schrödinger-Poisson solver based on the non-equilibrium Green’s functions formalism and in the framework of effective mass approximation. This allows studying the influence of quantum effects on electrical performance of ultra-scaled devices. We have implemented different mode-space methodologies in our 3D quantum-mechanical simulator and moreover introduced a new method to deal with discontinuities in the device structures which is much faster than the coupled-mode-space approach